1. Field of the Invention
The present invention relates to a method and apparatus for testing the electrostatic discharge sensitivity which will be called "ESD sensitivity" of a semiconductor device such as a memory element or the like.
2. Description of Related Art
The most general method for testing the ESD sensitivity of a semiconductor device, which will be called an "IC" herein, is, at present, a capacitor discharge testing method in which a human body model is taken into account. Such a method is shown in FIG. 10. Referring to FIG. 10, a high-voltage power source 81 charges a capacitor 82, the charge of which is applied to a terminal to be measured 85 when a switch 83 is actuated. At this time, the charge is discharged to ground through an electrostatic protection diode 87 in the IC 86 and earthline 88.
Such a testing method has publicly been standardized, for example, as U.S. Standard MIL-STD-883D, Method 3015 in which a capacitor must have a capacity C=100 pF and a discharge resistor 84 must have a resistance R=1.5 k .OMEGA.. Japanese EIAJ Standard sets C=200 pF and R=0 .OMEGA.. Further, a charging protection resistor 89 is set to range between 1M .OMEGA. and 10M .OMEGA..
There is also known another ESD testing method which is called "Charged Device Model test"(that will be called "CDM test" herein). FIG. 11 is a circuit diagram of the conventional CDM testing system. Unlike the human body model, the CDM testing system uses a parasitic capacitor having a metal electrode 92 in contact with the surface of an IC package and an IC chip electrode 93 in a package 91 of dielectric material. After the parasitic capacitor has been charged, it is discharged to ground via the earth line 88 through a switch 94 to estimate the ESD sensitivity of the IC.
Since the creation of static electricity highly depends on the environment, it is important that the ESD test will not be affected by circumstances such as temperature, humidity, testing machine used and so on. In other words, there is an important problem in that the discharge waveform should be uniform with better reproducibility. The testing method of the prior art uses a mercury relay as a switch for discharging a capacitor (which corresponds to the switch 83 in FIG. 10) to allow the standardization of buildup property in the discharge waveform by suppressing a chattering in the discharge waveform and also holding the wiring capacity and inductance component as low as possible. Since any relay has a stray capacitance ranging between several of pF's and several tens pF, however, the discharge waveform depends on the characteristics of the relay and is different from the electrostatic discharge actually encountered in the IC, that is, corona discharge or spark discharge. Thus, minute ICs developed in recent years tend to exhibit a sufficient strength in the publicly standardized ESD test, but have been found to be electrostatically defective in actual markets. The results estimated by the testing method of the prior art will not be acceptable in the markets and there is a need for an improved testing method and apparatus.